On Thu, Nov 18, 2021 at 11:47:51AM +0530, Ruchika Gupta wrote:
> Firmwares before U-Boot may be capable of doing tpm measurements
> and passing them to U-Boot in the form of eventlog. However there
> may be scenarios where the firmwares don't have TPM driver and
> are not capable of extending the measurements in the PCRs. To
> cater to such platforms, read the PCR0 to determine if the
> previous firmwares have extended the PCR0. If not, then extend
> the PCR's as the eventlog is parsed.
>
> Signed-off-by: Ruchika Gupta <ruchika.gu...@linaro.org>
> ---
> lib/efi_loader/efi_tcg2.c | 86 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 86 insertions(+)
>
> diff --git a/lib/efi_loader/efi_tcg2.c b/lib/efi_loader/efi_tcg2.c
> index c97766eae3..cbd0c7d224 100644
> --- a/lib/efi_loader/efi_tcg2.c
> +++ b/lib/efi_loader/efi_tcg2.c
> @@ -178,6 +178,43 @@ static efi_status_t tcg2_pcr_extend(struct udevice *dev,
> u32 pcr_index,
> return EFI_SUCCESS;
> }
>
> +/* tcg2_pcr_read - Read PCRs for a TPM2 device for a given tpml_digest_values
> + *
> + * @dev: device
> + * @digest_list: list of digest algorithms to extend
> + *
> + * @Return: status code
> + */
> +static efi_status_t tcg2_pcr_read(struct udevice *dev, u32 pcr_index,
> + struct tpml_digest_values *digest_list)
> +{
> + struct tpm_chip_priv *priv;
> + unsigned int updates, pcr_select_min;
> + u32 rc;
> + size_t i;
> +
> + priv = dev_get_uclass_priv(dev);
> + if (!priv)
> + return EFI_DEVICE_ERROR;
> +
> + pcr_select_min = priv->pcr_select_min;
> +
> + for (i = 0; i < digest_list->count; i++) {
> + u16 hash_alg = digest_list->digests[i].hash_alg;
> + u8 *digest = (u8 *)&digest_list->digests[i].digest;
> +
> + rc = tpm2_pcr_read(dev, pcr_index, pcr_select_min,
> + hash_alg, digest, alg_to_len(hash_alg),
> + &updates);
> + if (rc) {
> + EFI_PRINT("Failed to read PCR\n");
> + return EFI_DEVICE_ERROR;
> + }
> + }
> +
> + return EFI_SUCCESS;
> +}
> +
> /* tcg2_agile_log_append - Append an agile event to out eventlog
> *
> * @pcr_index: PCR index
> @@ -1488,10 +1525,12 @@ static efi_status_t efi_init_event_log(struct udevice
> *dev)
> struct tcg_pcr_event *event_header = NULL;
> struct tpml_digest_values digest_list;
> size_t spec_event_size;
> + bool extend_pcr = false;
> efi_status_t ret;
> u32 pcr, pos;
> u64 base;
> u32 sz;
> + int i;
>
> ret = platform_get_tpm2_device(&dev);
> if (ret != EFI_SUCCESS)
> @@ -1541,6 +1580,26 @@ static efi_status_t efi_init_event_log(struct udevice
> *dev)
> goto free_pool;
> }
>
> + ret = tcg2_pcr_read(dev, 0, &digest_list);
> + if (ret) {
> + log_err("Error reading PCR 0\n");
> + goto free_pool;
> + }
> +
> + /*
> + * If PCR0 is 0, previous firmware didn't have the capability
> + * to extend the PCR. In this scenario, extend the PCR as
> + * the eventlog is parsed.
> + */
> + for (i = 0; i < digest_list.count; i++) {
> + u8 buffer[TPM2_DIGEST_LEN] = { 0 };
> + u16 hash_alg = digest_list.digests[i].hash_alg;
> +
> + if (!memcmp((u8 *)&digest_list.digests[i].digest,
> + buffer, alg_to_len(hash_alg)))
> + extend_pcr = true;
> + }
> +
> while (pos < sz) {
> ret = tcg2_parse_event(dev, buffer, sz, &pos,
> &digest_list, &pcr);
> @@ -1548,6 +1607,33 @@ static efi_status_t efi_init_event_log(struct udevice
> *dev)
> log_err("Error parsing event\n");
> goto free_pool;
> }
> +
> + if (pcr != 0) {
> + /*
> + * Eventlog passed by firmware should extend
> + * PCR0 only.
I don't get this comment. The previous stage bootloader could extend any
PCR (0-7) as long as it adheres to the PC client spec right?
I think we just need the logic below here, were we decide to start with an
empty eventlog if the prior stage boot loader doesn't expose one.
> + */
> + log_err("Invalid PCR\n");
> + goto free_pool;
> + }
> +
> + if (extend_pcr) {
> + ret = tcg2_pcr_extend(dev, pcr, &digest_list);
> + if (ret != EFI_SUCCESS) {
> + log_err("Error in extending PCR\n");
> + goto free_pool;
> + }
> +
> + /* Clear the digest for next event */
> + for (i = 0; i < digest_list.count; i++) {
> + u16 hash_alg =
> + digest_list.digests[i].hash_alg;
> + u8 *digest =
> + (u8 *)&digest_list.digests[i].digest;
> +
> + memset(digest, 0, alg_to_len(hash_alg));
> + }
> + }
> }
>
> memcpy(event_log.buffer, buffer, sz);
> --
> 2.25.1
>
Cheers
/Ilias
